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<head>
  <meta charset="UTF-8">
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  <title>Document</title>
  <style>
    div {
      width: 400px;
      height: 400px;
      /* 1、背景颜色 */
      background-color: red;
      /* 透明取值 */
      /* background-color: rgba(0,0,0,0); */
      /* background-color: transparent; */
    }
  </style>
</head>
<body>
  <div></div>
</body>
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